Ten children with either Sillence type III or IV Osteogenesis Imperfecta have participated in a randomized prospective study evaluating the discontinuation of long leg braces. Each patient was paired with a partner determined to have similar clinical characteristics. Within each of 5 pairs, one child received braces for 16 months and the other did not. Then the assignment was switched. The children were evaluated every four months at NIH. At each admission they received the following: measures of height, weight, bone density (vertebral body, femur, radius), X-rays of long bones of lower extremity and spine, muscle strength testing (MMT) videobased gait analysis, range of motion (ROM). In addition, we have received monthly information completed by parents, describing each child's activities for a two week period. Preliminary data analysis suggests that children who are braced are engaged in more moderately strenuous activity than those who are unbraced (p.009) while unbraced children are minimally active (p.09). Children have less disability (clinical severity) during braced periods than during unbraced periods as determined by a functional activity level score (p.05). Statistical analysis was performed using a combined two sample Wilcoxon Sign Rank Test. Numbers of fractures incurred were compared for braced and unbraced periods. More fractures occurred during the unbraced interval, but this did not reach statistical significance (p=.21).